1. Field of the Invention
The present invention relates to a steel for a vehicle suspension spring such as a coil spring, a torsion bar or a laminated leaf spring, which has a good sag-resistance, a good fatigue resistance and a good hardenability.
2. Description of the Prior Art
There has been an increasing demand for light weight suspension springs reflecting a trend for light weight automobiles, in recent years. As an attempt to meet such a demand, it is said to be an effective approach to the reduction of weight to design the springs to have an increased stress and to use them under a high stress state.
However, if presently available spring steels are used under a high stress condition, there will be problems such that the durability will be reduced and the sagging will be increased, and consequently the level of the springs will be lowered, accordingly, the height of the vehicle will be decreased and the location of the bumper will be lowered thus leading to a serious problem for safety.
Under the circumstances, there is a strong demand for a spring steel having a good sag-resistance, which makes high stress designing possible. However, there are severe requirements for light weight suspension springs. Accordingly, it has been strongly desired to develop a spring steel having a sag-resistance superior to that of SAE 9260.
Further, in the case of a thick coil spring or a thick torsion bar made of a wire having a diameter of at least 20 mm, or a laminated leaf spring made of a sheet material having a thickness of at least 12 mm which is used for a relatively large automobile, it is difficult to harden the material to its core portion during the heat treatment, and the structure of the core portion tend to be bainite or ferrite-pearlite which has a lower hardness than a martensite structure, thus giving an inferior sag-registance.
Thus, it is desired to provide a steel for springs, which, even in a form of a thick coil spring, a thick torsion bar or a thick leaf spring, is capable of forming a martensite structure extending to the core portion by the heat treatment, and thus providing a good hardenability without a loss of a sag-resistance.